This invention relates to a simplified, highly efficient, pulse width modulated inverter system for supplying single phase, adjustable amplitude and frequency sinusoidal voltage to a single phase a-c induction motor at a constant amplitude/frequency ratio.
When an inverter is used to drive a motor at a variable speed, some arrangement is usually employed to vary the voltage applied to the motor in direct proportion to frequency, or speed, changes so that the ratio of the motor voltage to frequency will always remain the same. A fixed or constant amplitude/frequency ratio is desirable for a variety of reasons, such as to avoid overheating of the motor and to provide the motor with a constant torque output capability regardless of motor speed. When the inverter system is of the pulse width modulated type, complex circuitry is generally needed to maintain a constant ratio of voltage to frequency.
In contrast, the present invention provides a relatively simple, inexpensive, and efficient circuit arrangement for obtaining the desired constant amplitude/frequency ratio for the motor voltage in a pulse width modulated inverter system. The invention is particularly attractive when incorporated in such an inverter system for driving a single phase a-c induction motor.